1. Field of the Invention
The present invention relates generally to the fields of cancer biology and drug delivery and, more specifically, to peptides that selectively home to a tumor, particularly a malignant tumor, to compositions comprising an agent such as a therapeutic agent conjugated to such tumor homing molecules, and to methods of using such molecules to target an agent to a tumor.
2. Background Information
Continuous developments over the past quarter century have resulted in substantial improvements in the ability of a physician to diagnose a cancer in a patient. For example, antibody based assays such as that for prostate specific antigen now allow early diagnosis of cancers such as prostate cancer. More recently, methods of genetic screening are becoming available to identify persons that may be particularly susceptible to developing a cancer. Genetic screening methods are based on the identification of one or more mutations in a gene that correlates with the development of a cancer. For example, the identification of genes such as BRCA1 and BRCA2 allowed the further identification of mutations in these genes that, in some cases, can correlate with susceptibility to developing breast cancer.
Unfortunately, methods for treating cancer have not kept pace with those for diagnosing the disease. Thus, while the death rate from various cancers has decreased due to the ability of a physician to detect the disease at an earlier stage, the ability to treat patients presenting with more advanced disease has advanced only minimally.
A major hurdle to advances in treating cancer is the relative lack of agents that can selectively target the cancer, while sparing normal tissue. For example, radiation therapy and surgery, which generally are localized treatments, can cause substantial damage to normal tissue in the treatment field, resulting in scarring and, in severe cases, loss of function of the normal tissue. Chemotherapy, in comparison, which generally is administered systemically, can cause substantial damage to organs such as bone marrow, mucosae, skin and the small intestine, which undergo rapid cell turnover and continuous cell division. As a result, undesirable side effects such as nausea, loss of hair and drop in blood cell count occur as a result of systemically treating a cancer patient with chemotherapeutic agents. Such undesirable side effects often limit the amount of a treatment that can be administered. Thus, cancer remains a leading cause of patient morbidity. and death.
Efforts have been made to increase the target specificity of various drugs. For example, where a unique cell surface marker is expressed by a population of cells making up a tumor, an antibody can be raised against the unique marker and a drug can be linked to the antibody. Upon administration of the drug/antibody complex to the patient, the binding of the antibody to the marker results in the delivery of a relatively high concentration of the drug to the tumor. Similar methods can be used where a particular cancer cell or the supporting cell or matrix expresses a unique cell surface receptor or a ligand for a particular receptor. In these cases, the drug can be linked to the specific ligand or to the receptor, respectively, thus providing a means to deliver a relatively high concentration of the drug to the tumor.
Tumors are characterized, in part, by a relatively high level of active angiogenesis, resulting in the continual formation of new blood vessels to support the growing tumor. Such angiogenic blood vessels are distinguishable from mature vasculature. One of the distinguishing features of angiogenic vasculature is that unique endothelial cell surface markers are expressed. Thus, the blood vessels in a tumor provide a potential target for directing a chemotherapeutic agent to the tumor, thereby reducing the likelihood that the agent will kill sensitive normal tissues. Furthermore, if agents that target the angiogenic blood vessels in a tumor can be identified, there is a likelihood that the agents can be useful against a variety of different types of tumors, since it is the target molecules in the angiogenic vessels that are recognized by such agents and not receptors specific for the tumor cells. However, the use of molecules that can bind specifically to tumor vasculature and target a chemotherapeutic agent to the tumor has not been demonstrated.
While linking a drug to a molecule that homes to a tumor can provide significant advantages for treatment over the use of a drug, alone, use of this method is severely limited by the scarcity of useful cell surface markers expressed in a tumor. Thus, a need exists to identify molecules that can selectively home to a tumor, particularly to the vasculature supporting the tumor. The present invention satisfies this need and provides related advantages as well.
The present invention relates to molecules that selectively home to tumors, generally to the vasculature supporting the tumor. For example, the invention provides tumor homing peptides that contain, for example, the motif asparagine-glycine-arginine (NGR) or glycine-serine-leucine (GSL), or the xcex1v-containing integrin binding motif, arginine-glycine-aspartic acid (RGD).
The invention also relates to compositions comprising a tumor homing molecule, such as a tumor homing peptide, linked to a moiety to produce a tumor homing molecule/moiety conjugate. Such a moiety can be a drug, for example, a cancer therapeutic agent such as doxorubicin, taxol, cis-platinum, or the like, in which case the tumor homing molecule/moiety conjugate provides a therapeutic reagent. A moiety conjugated to a tumor homing molecule also can be a detectable label, for example, a radionuclide or paramagnetic spin label, such that the molecule/moiety conjugate provides a diagnostic reagent.
The invention also relates to methods of targeting a moiety such as a drug to a tumor by contacting the tumor homing molecule/moiety conjugate with the tumor. Thus, the invention provides methods of diagnosing or treating a cancer in a subject by administering a composition comprising a tumor homing molecule conjugated to a cancer therapeutic agent to the subject. For example, administration of a composition comprising a doxorubicin/CDCRGDCFC (SEQ ID NO: 1) conjugate to a mouse bearing a transplanted breast carcinoma substantially reduced the growth of the breast cancer and the number of metastases and resulted in substantially greater survival as compared to tumor bearing mice treated with doxorubicin, alone, or with doxorubicin conjugated to an unrelated peptide.
The invention further relates to methods of identifying a target molecule in a tumor by detecting selective binding of the target molecule to a tumor homing molecule. For example, a peptide that selectively homes to a tumor can be attached to a solid matrix for use in affinity chromatography. A sample of the tumor can be obtained and passed over the affinity matrix under conditions that allow specific binding of the target molecule, which then can be collected and identified using well known biochemical methods. Thus, the invention also provides a target molecule, which acts as a receptor for a tumor homing molecule. Such a target molecule can be useful, for example, for raising an antibody specific for the target molecule.